We study a model neuronal circadian oscillator system, the eye of the mollusc, Aplysia california, that can be maintained in vitro for up to 2 weeks. Extracellular recordings indicate that the eye, in continuous darkness and at constant temperature, produces a circadian oscillation of the frequency of compound action potentials in the optic nerve that is transmitted, in the intact animal, to the cerebral ganglion. We are investigating the nature of this neuronal circadian oscillator system by pharmacological manipulation, biochemical analyses, morphological (light and electron-microscopic) approaches and by intracellular recording and staining of cells within the eye. In addition we have found that we can use X-rays or near U-V light to selectively block the circadian oscillator function without damaging other functions in the eye and are investigating the mechanisms of this blockade. The behavioral functions of the circadian oscillator in the eye are being investigated by quantitating locomotor movements in intact freely-moving Aplysia with specific lesions placed in the CNS. Finally, the neural flow of circadian information, generated by the eyes, is being traced to the pedal locomotor oscillators in reduced preparations (eyes and head ganglia).